The present invention is in the field of whisker-reinforced composite ceramic articles, and specifically relates to an improved method for making whisker reinforced composites which permits the uniform incorporation of small or relatively large proportions of reinforcing whisker materials in the ceramic matrix.
The use of inorganic whiskers and fibers to reinforce glasses, glass ceramics, and ceramics has long been practiced. In most references in the literature, whiskers have frequently been characterized as relatively short, single-crystal fibers of small (less than 100 microns) diameter, while fibers are considered to be multicrystalline or amorphous and are generally sufficiently long to be used in woven or otherwise interlocking bundles, tows or cloth. Hence whiskers are typically incorporated as a randomly dispersed phase in a selected glass or ceranic matrix, while fibers are more frequently incorporated in a controlled oriented or interlocking alignment.
The mechanism of strengthening of glass or ceramic bodies by fibers is considered to be that of load transfer by the matrix to the fibers through shear. This load transfer shifts stress from the glass or ceramic matrix to the relatively long, high modulus fibers, while the fibers at the same time may act to impede crack propagation in the matrix material.
Whiskers are thought to impart strengthening by a similar mechanism, but load transfer to whiskers by the matrix is more limited due to the limited length and aspect ratio of the whiskers. Theoretically, a whisker which is sufficiently short will not be loaded to the breaking point by the matrix under stress, and therefore full advantage cannot be taken of the high strength of the whiskers.
Among the fibers and whiskers which have been suggested for use as reinforcement for nonmetal matrix materials are silicon carbide, silicon nitride, alumina and carbon whiskers. For example, U.S. Pat. No. 4,324,843 describes SiC fiber reinforced glass-ceramic composite bodies wherein the glass-ceramic matrix is of aluminosilicate composition. U.S. Pat. No. 4,464,475 describes similarly reinforced glass-ceramics comprising barium osumilite as the predominant crystal phase, while U.S. Pat. No. 4,464,192. describes whisker-reinforced glass-ceramic composites of aluminosilicate composition.
Composite materials have also been proposed for applications requiring strength and stability at very high temperatures. In this case, however, wholly ceramic materials, i.e., crystalline materials essentially free of residual glassy phases, are considered to hold the greatest promise. Ceramic materials are more difficult to reinforce with fibers or whiskers than materials comprising glassy phases, and the properties of whisker-ceramic composites are not as well understood, but they offer the prospect of high temperature creep resistance. Hence, it is presently thought that whiskers dispersed in a crystalline matrix will mainly occupy sites along grain boundaries in the material, and will thereby improve high-temperature stability by increasing the length of shear required and/or the complexity of the shear needed to cause permanent dimensional change or creep in the material.
A further objective of whisker reinforcement in ceramic and glass-ceramic materials for high temperature applications is that of increasing the toughness of the material. A toughened ceramic material exhibits improved resistance to cracking failure from flaws sustained in use, offering the possibility of increased fatigue lifetime and, most desirably, a noncatastrophic mode of failure which can be more easily identified by routine inspection.
The mechanisms of toughening in wholly ceramic matrices have been reviewed by R. W. Rice in "Mechanisms of Toughening in Ceramic Composites", Ceram. Eng. Sci. Proc., 2 (7-8) 661-701 (1981). Strengthening mechanisms identified in this monograph include load transfer, prestressing, crack impediment or deflection, and fiber pullout. U.S. Pat. No. 4,543,345 reports on the addition of silicon carbide whiskers to ceramic matrix materials such as alumina, mullite and boron carbide, with some toughening observed to result from these additions. Thus whisker toughening has been demonstrated to offer useful enhancements to the physical properties of ceramics in some systems.
In order to fully realize the potential of glass and ceramic composite materials for the production of strong tough components, the materials themselves must be produced in a form which is free of extraneous glassy or crystalline phases, and which is also free of included cracks or voids. Particularly in the case of ceramic and glass-ceramic composite materials, these requirements place constraints on the composition and morphology of the starting materials. For example, pure matrix materials must be available in a readily dispersible particulate form so that whisker reinforcement additives can be uniformly dispersed in the matrix prior to consolidation.
It is difficult to produce refractory compositions in high purity and small uniform particle size by melting. Further, even with hot-pressing at high temperatures, it is not easy to achieve complete consolidation to a void-free product unless an optimum particle size distribution in the matrix material has been achieved. Finally, the presence of a whisker phase significantly increases the viscosity of the material being consolidated, such that the proportions of whisker reinforcement in the matrix must generally be limited to moderate levels (less than about 50 volume percent) where full, void-free consolidation is required.
It is a principal object of the present invention to provide an improved method for producing whisker-reinforced composite materials which facilitates the production of dense homogeneous composites at higher whisker loadings and lower temperatures and pressures than previously attainable.
It is a further object of the invention to provide composites of enhanced purity and a method for making them.
It is a further object of the invention to provide whisker-reinforced composites incorporating highly refractory matrix materials which are nevertheless consolidated to nearly theoretical density.
It is a further object of the invention to provide composites exhibiting enhanced uniformity of whiskers in the matrix and thus properties which are more reproducible and homogeneous.
Other objects and advantages of the invention will become apparent from the following description thereof.